In the late 1980s/early 1990s, late night television was exciting. It was unpredictable, innovative, and buzz-worthy. Today, late night TV (Leno, Conan, Fallon, Letterman, etc.) has lost its charm. I'm not sure when it happened, but I am sure that this form of entertainment has lost most of its significance as an important cultural movement.
The expanded variety of entertainment that comes from cable television and the Internet has probably diluted the reach of late night. But I believe the main cause for the decline in relevance stems from a decline in the taboo-factor. No longer is network television-based late night comedy the most exciting form of nearly free entertainment. No longer is any unpredictability associated with the corporate machine that drives the typical plug-the-movie/album format of these stale shows.